False-positive filtering (emails incorrectly identified as spam) remains high among leading email service providers (ESPs), including Hotmail and Gmail.

The false-positive filtering rate for Gmail nevertheless improved dramatically in the second quarter, with only 2.97 percent of emails falsely identified as spam, compared with the previous quarter's 44 percent, according to a study (pdf) by email marketing solutions provider Lyris Technologies. However, Hotmail's false-positive filtering, though improving, remains high - 18.2 percent in Q2, compared with 23.4 percent in Q1.

Nevertheless, U.S. ESPs' rates overall improved approximately 4 percent for both gross and inbox deliverability compared with Q1 rates. "While false positives are increasing among some ESPs, the industry as a whole is winning the fight to reduce the amount of spam," says Dave Dabbah, Director of Sales and Marketing, Lyris Technologies.

Users with addresses with one of the top 10 U.S. ESPs were 27 percent more likely to receive opt-in email in their inbox than those who used one of the bottom 10 providers: 97.8 percent versus 70.6 percent. Still, that rate for the bottom 10 is an improvement of 11.2 percentage points in deliverability compared with Q1.

False-positive spam filtering among European ISPs remains lower, achieving an average rate of only 0.075 percent compared with the U.S. average of 3.29 percent. This disparity is again due in part to excessive false-positive filtering at two ISPs - cs.com (Compuserve.com) and iwon.com - and U.S. ISPs' and ESPs' more stringent filtering of unsolicited emails.